We have developed several methods to study platelet activation ex-vivo and in-vitro. These studies were made possible by the development and implementation of techniques to isolate platelets from human blood without inducing platelet activation or secretion. We have developed nine (9) monoclonal antibodies which react only with activated platelets. Our results have shown that in some forms of von Willebrand's disease platelet activation if present and causes thrombocytopenia. In patients with arteriosclerotic heart disease after exercise platelet activation is increased. This suggests that some degree of platelet activation occurs in these patients following strenuous exercise. In volunteers receiving endotoxin, spontaneous platelet activation or enhanced agonist- induced platelet activation is not observed. Exercise in individuals with minor or mild atherosclerosis does not induce platelet activation. We have identified in patients with paroxysmal nocturnal hemoglobinuria (PNH) platelet activation. The activation was determined by release of alpha granule proteins and the increased fibrinogen binding to the platelet surface. No evidence of coagulation activation was observed. These findings suggest that platelet activation may be the primary mechanism of venous thrombosis in PNH. Studies to test this hypothesis are in the process of being implemented. We have also identified moducation of alpha- granule proteinuria on platelets activated in-vitro.